In general, in amplifiers, such as high power amplifiers (HPAs), provided in, for example, transmission devices, when output electrical power approaches a saturation region due to an increase in input electrical power, the linearity of the input electrical power and the output electrical power is lost. Consequently, when an amplifier is operated in the vicinity of the saturation region in which the efficiency of the electrical power is high, nonlinear distortion is generated. Because the nonlinear distortion causes out-of-band radiation or the like, in order to suppress the nonlinear distortion, it is conceivable to use a method of converting a signal amplified by an amplifier, thereby obtaining the constant amplitude.
Specifically, studies have been conducted on a method in which a transmission signal that is to be amplified by the amplifier is converted to a signal with the constant amplitude by performing, for example, ΣΔ modulation and the converted signal is amplified by the amplifier and then transmitted. With the ΣΔ modulation, because a signal with a multi-bit quantization bit rate is converted to a signal with a 1-bit quantization bit rate, the transmission signal in which the amplitude varies can be converted to a transmission signal in which the amplitude is constant. Furthermore, because the amplitude of the transmission signal becomes constant, the electrical power that is input to the amplifier becomes constant and thus the occurrence of nonlinear distortion can be suppressed.
However, when a signal is subjected to the ΣΔ modulation, quantization noise with a relatively high level is generated outside the frequency band of the signal. Accordingly, in order to remove the quantization noise, a band-pass filter with a high Q value is sometimes provided downstream of the amplifier. Furthermore, if the band-pass filter with the high Q value is provided, the size and the cost of transmission devices are increased. Accordingly, a study has been conducted on a method, in which, in addition to a path for amplifying a transmission signal, a path for amplifying a noise component is provided in a transmission device and a noise component is removed from the amplified transmission signal.
FIG. 8 is a block diagram illustrating an example of a transmission circuit. The transmission circuit illustrated in FIG. 8 includes a signal processing unit 10, a first signal generating unit 20, a second signal generating unit 30, amplifiers 40 and 50, and a combining unit 60.
In the transmission circuit illustrated in FIG. 8, when a transmission signal subjected to signal processing by the signal processing unit 10 is input to the first signal generating unit 20 and the second signal generating unit 30, the first signal generating unit 20 performs, for example, ΣΔ modulation on the transmission signal. At this time, a noise component generated due to the ΣΔ modulation is added to the transmission signal. Furthermore, the second signal generating unit 30 generates the same noise component as that generated due to the ΣΔ modulation performed by the first signal generating unit 20. Then, transmission signal that has been subjected to the ΣΔ modulation by the first signal generating unit 20 is amplified by the amplifier 40, whereas the noise component generated in the second signal generating unit 30 is amplified by the amplifier 50. The amplified transmission signal and the amplified noise component are combined by the combining unit 60. At this time, by inverting the phase of the noise component amplified by the amplifier 50 and by combining the noise component with the transmission signal, the noise component is removed from the transmission signal.
[Patent document 1] Japanese Laid-open Patent Publication No. 2004-72735.
[Patent document 2] Japanese Laid-open Patent Publication No. 2005-295521.
[Non-Patent Document 1] Antoine Frappe et al. “An All-Digital RF Signal Generator Using High-Speed ΣΔ Modulators” IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 44, No. 10, October, 2009.
[Non-Patent Document 2] Toru Matsuura et al. “A High Efficiency Transmitter with a Delta-Sigma Modulator and a Noise Cancellation Circuit” European Conference on Wireless Technology, 2004.
[Non-Patent Document 3] Hsin-Hung Chen et al. “Joint Polynomial and Look-Up-Table Predistortion Power Amplifier Linearization” IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS-II: EXPRESS BRIEFS, Vol. 53, No. 8, August 2006.
However, with the technology that removes the noise component by amplifying the transmission signal and the noise component using different paths and by combining the transmission signal with the noise component, there is a problem in that, when the characteristics of both paths are unbalanced, it is difficult to sufficiently remove the noise component. Specifically, for example, in the transmission circuit illustrated in FIG. 8, when the amplifiers 40 and 50 have different linear characteristics or gains, the transmission signal and the noise component are not amplified under the same condition and, even when the amplified transmission signal and the amplified noise component are combined, the noise component is not sufficiently removed. Accordingly, the characteristics of the paths used for amplifying the transmission signal and the noise component are preferably be matched; however, because there are individual differences in components, such as amplifiers, provided in both the paths, it is not practical to completely match the characteristics of both the paths.